The deal, signed last week, at the very least has the two factions, the largest in the Syrian rebellion, agreeing not to fight against one another, and to work together to some length against their mutual enemies, which is to say both the US and Syrian governments.

Previous agreements, including them along with other rebel factions, have amounted only to a basic nonaggression pact, though this appears to include more than a little open cooperation on top of that.

Before the US launched its air war in Syria in September, ISIS and Nusra were mortal enemies. The US strikes targeting both sides, however, have them convinced they have a mutual enemy that is more pressing than their own fighting.